List of Abstracts 2024: PLENARY SESSION

Nickel-Titanium shape memory alloys (NT-SMAs) have garnered both scientific and technological interest over several decades. They exhibit to a stress and temperature-dependent solid state transformation that converts thermal energy into mechanical work and vice versa. This facilitates actuators with relatively large instantaneous energy density compared to more conventional actuation methods such as electric motors. A challenge, however, is to train NT-SMAs to follow repeatable thermal-stress actuation history over numerous cycles. This depends on dislocation substructure, precipitates, and residual stress spanning from intergranular to nanometer scales.

This talk describes efforts to make, characterize, and model NT-SMAs that are synthesized by laser powder bed fusion additive manufacturing (LBPF) and to process NT-SMAs at strain rates exceeding 104/s. The large cooling rates and strain rates can produce microstructures, textures, residual stress, and mechanical response that are difficult to achieve through conventional processing. For LPBF, the texture can be modulated from <001> dominated to more random orientations. This can be used to control the training and performance characteristics by selecting grain orientations with yield-before-transform or transform-before-yield characteristics. LPBF combined with post-heat treatment imparts extraordinary shape memory performance that is attributed to nm-to-micron-scale bimodal grain sizes distributions. For comparison, high rate deformation is achieved through impact joining of NT-SMA to brass and through Taylor impact experiments. These methods can increase nanoindentation hardness by 50% or greater while maintaining recoverable deformation. They also produce nonuniform grain refinement down to <10 nm. Together, the LPBF and high strain rate processing offer the potential to achieve superior work output and functional stability through spatial distributions in chemistry, texture, and grain size.

Today it is widely known that the mechanical behaviour of engineering materials depends on a variety of loading conditions, such as deformation, deformation rate, temperature and stress state. In order to being able to use materials in a targeted manner according to their performance, it is therefore necessary to know the material behaviour in precisely those loading ranges in which they are to deployed. While standardised testing procedures, such as quasi-static tensile tests, hardness measurements or notched bar impact tests, are justified in the area of quality assurance and process monitoring, they are hardly suitable for assessing the performance of a material in later use. Therefore, specialised experimental methods are required to provide the necessary findings: material testing at high stress rates.
The presentation will give a brief outline of the history and development of materials testing at high rates. Basic and origin concepts will be presented and discussed. Based on that, modern testing technologies will be presented and selected examples from the automotive, aerospace, production engineering and safety engineering sectors will be used to show how detailed knowledge of material behaviour can sustainably improve product development. This includes a short journey through constitutive material modelling and numerical simulation as well as a view into future challenge

The layer-by-layer approach of metal powder bed based additive manufacturing opens a fully new design freedom in manufacturing: Voxel based material design, i.e., the control and adaption of the local material properties as needed. The microstructure can be adjusted via the solidification conditions and the local composition. In addition, the layer-by-layer process also enables the components to be inspected during the build. The electron beam can be used in a variety of ways, as an energy source to maintain the building temperature, to solidify the material and also, similar to the scanning electron microscope, for process observation.
As part of this talk, additive manufacturing with the electron beam will be introduced and its specific properties will be discussed. The challenges and opportunities in electron-optical process observation are presented. Two examples for material design are shown. The first example demonstrates how mastering local selective evaporation can locally change the composition, microstructure and ultimately properties of titanium aluminides. The second example shows how the texture and grain size of nickel-based superalloys can be mastered.